The International Astronomical Union has spoken: we've got 12 planets, with …

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Humans like to think in binary terms. Something is true or false. You're either with us or against us. But the natural world is a place of fuzzy borders and indistinct boundaries, where many things are distributed along a spectrum, rather than at distinct poles. For example, it's nice to think that we can easily define a species as the ability to produce fertile hybrids until we find that this definition means that polar and grizzly bears are the same species. In the end, nature often makes clear that our attempts at definition are little more than human constructs, layered on top of a very messy reality.

This bit of pontification was brought about by the International Astronomical Union's attempts to define exactly what is or is not a planet. The results are in, along with a detailed description of them. The official word is:

A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

Which seems simple and quantitative, but this still apparently needed two footnotes—one larger than the definition itself—and three more paragraphs (with two more footnotes) for clarity. I'm not convinced this provides clarity, as it immediately subdivides planets into "classical planets" and a new class called "Plutons", which unsurprisingly contains Pluto and everything further out in the same size class. But it's the things at Pluto's distance and closer that are the big surprises. Pluto's former moon Charon now joins it as a Pluton. The two objects are so similar in mass that they both orbit their shared center of mass somewhere in between them; neither orbits a planet, so they're both in. Also moving up in the world is Ceres, the largest asteroid, which is apparently sufficiently massive to have pulled itself in towards a spherical shape. Meanwhile, a dozen more small bodies have been placed on a potential planet "watchlist," awaiting observations as to whether they're spherical or not.

The definition will be discussed during the IAU's meeting in Prague this week, and voted on by its general assembly on the 24th. But it's already clear that not everyone is happy with the results (choice quote out of Cal Tech: "It's a mess"). When we mentioned that this decision was imminent, it was suggested that it was a good example of how science can reevaluate its conclusions in light of better data. But the new definition seems to be an awkward compromise, one which keeps everything we "know" is a planet, but creates an entirely new class of planets in order to keep Pluto in the group. In doing so, it pulls in many objects that I'm not sure anyone expected to be a planet. Lest this seem to be nothing more than wrangling over semantics, let me point out that a discussion of the planets is a fundamental part of the science education in most schools. Should the resolution pass, it will make most of the science textbooks that include the solar system obsolete overnight.